1. Field of the Invention
The invention relates to a process for producing a hollow article made of a laminated composite material consisting of reinforcing fibres embedded in a polymerized organic resin matrix, and more particularly to such a process for producing an article having high strength, accuracy and temperature resistance characteristics.
Laminated composite materials comprising reinforcing fibres embedded in a matrix of polymerized resin are particularly useful in the aeronautical industry because of their excellent strength-to-weight ratio, and there is an increasing tendency to use such materials instead of metal alloys whenever possible, particularly in the case of thin-walled articles conventionally made by casting or metal fabrication techniques.
Endeavours are therefore being made to produce turbomachine parts, especially parts for aircraft turbojet engines, such as casing arms for low pressure compressors or hollow low pressure compressor blades having thin walls defining and extending around cavities which have an opening, i.e. which are not fully closed.
These articles must be integral in order to be free from weakening assembly zones. The articles must also be accurate and have a good surface texture so as to avoid the need for subsequent machining. Furthermore, the articles must be able to withstand high temperatures and their cost must be comparable with or below the equivalent metal articles.
2. Summary of the Prior Art
In the known resin transfer moulding process, known as RTM, reinforcing fibres are placed in a mould having the shape of the finished article, very liquid resin is injected under pressure into the mould and the resin is polymerized while maintaining the pressure. This process enables strong accurate articles to be obtained in a wide variety of shapes. However, the resins used have poor temperature resistance, which limits the use of the process to articles which will remain cool.
Resins which can withstand higher temperatures are not sufficiently fluid before polymerization. Consequently, to produce laminated hollow articles with such resins it is necessary to:
pre-impregnate layers of fabrics or fibres with the resin; PA1 form a core which may or may not be destructible; PA1 surround the core by an inflatable elastomeric bladder; PA1 drape the pre-impregnated layers of fabric or fibres around the assembly of the core and the bladder; PA1 place the assembly of the core, the bladder and the pre-impregnated layers of fabric or fibres in a mould corresponding to the external shape of the finished article; PA1 inflate the bladder; PA1 polymerize the resin; PA1 deflate the bladder and remove the article from the mould; PA1 withdraw or destroy the core; and PA1 withdraw the bladder. PA1 a) providing a mould having a pair of walls for shaping said external surfaces of the article to be produced, a female part supporting one of said walls, a male part supporting the other of said walls and slidable in said female part to move said walls towards each other, and abutment surfaces on said female part and said male part which are engageable to define accurately the limit positions of said walls when moved towards each other; PA1 b) making at least one core corresponding to the shape of said at least one cavity in the article to be produced, said at least one core being made from a thermally expansible silicone elastomer; PA1 c) draping said at least one core with at least one layer of reinforcing fibres impregnated with hot polymerizable resin for forming said composite material; PA1 d) placing the assembly consisting of said at least one core and said at least one layer of resin-impregnated fibres in said mould; PA1 e) sliding said male part of said mould in said female part to move said walls towards each other and thereby compress the resin-impregnated fibre layers between said at least one core and said walls so as to shape said internal and external surfaces of the article simultaneously and to produce sufficient flow of the resin during polymerization; PA1 f) raising the temperature of said assembly in said mould to polymerize said resin and to expand said at least one core and thereby stretch said reinforcing fibres during the compression shaping of said at least one layer of resin-impregnated fibres and the polymerization of said resin; PA1 g) releasing the moulded article from said mould; and PA1 h) withdrawing said at least one core from said moulded article.
In this process inflating the bladder makes it possible simultaneously to press the resin impregnated layers of fabric which form the composite material against the mould wall, and to compress and cause flow of the material so as to reduce porosities due to air bubbles trapped between the fibre layers, reduce emissions of gas from the resin during polymerization, and expel the excess resin and thereby increase fibre density. A compression corresponding to 20% of wall thickness is usually achieved.
Clearly, in such a process only the article surface in contact with the mould wall is accurate, whereas the surface in contact with the bladder is irregular and rough and follows the inevitable heterogeneities of the draping of the reinforcing fibres. It might be conceivable to compress the composite material on the core, but this solution would cause unacceptable creasing of the reinforcing fibres, causing a reduction in the strength of the article.
A first problem is therefore to produce, from pre-impregnated fibres or fabrics, hollow articles of a variety of shapes whose internal and external surfaces are accurate and smooth, without creasing the reinforcing fibres.
Polymerization of the resin is accompanied by an emission of gaseous components and a reduction in the volume of the resin, both of these phenomena tending to make the resulting composite material porous. This porosity can be reduced, but not eliminated, by the use of gas removal means and by compressing the composite material before the resin hardens in the course of its polymerization, the compression preferably causing a substantial deformation or flow of the composite material. Since the residual porosity reduces the strength of the final article, a second problem is to achieve a general reduction in the porosity of the composite material and, to this end in particular, to increase flow of the composite material during polymerization.
When the cavities open to the exterior through openings which are too small, as is often the case, the cores can be eliminated only by destruction of the material of which they are made. Materials are on the market which can be moulded to the required shape, then dissolved by water or a solvent after the article has been moulded. However, such cores are unsuitable in the present case since to produce accurate internal surfaces there would need to be an inward compression of the composite material on the core, with the disadvantages previously described. Consequently, a third problem is to remove the cores after the moulding of the article.
The use of composite materials is also hampered by the high production cost of the articles as compared with equivalent metal alloy articles. The high cost is due in particular to the many manipulations required in the production process. The complexity of the manufacturing process should therefore not be increased.
French Patent No. 2562834 discloses a process for moulding hollow articles made of a composite fibre and polymerized resin material using an external mould and a core made of a silicone elastomer, this latter material having a very high thermal expansion coefficient. In this process the core compresses the composite material against the mould walls as a result of the thermal expansion of the silicone elastomer during the hot polymerization cycle. The process provides an article having accurate internal and external surfaces but the composite material of the article has appreciable porosity. Therefore, and in order to produce homogeneous and smooth surfaces, French patent No. 2562834 also proposes, at lines 1 to 5 on page 8, to use a paint or gelcoat.